Engines may be configured to operate with a variable number of active or deactivated cylinders to increase fuel economy, while optionally maintaining the overall exhaust mixture air-fuel ratio about stoichiometry. Such engines are known as variable displacement engines (VDE). Therein, a portion of an engine's cylinders may be disabled during selected conditions defined by parameters such as a speed/load window, as well as various other operating conditions including vehicle speed. A VDE control system may disable a selected group of cylinders, such as a bank of cylinders, through the control of a plurality of cylinder valve deactivators that affect the operation of the cylinder's intake and exhaust valves, or through the control of a plurality of selectively deactivatable fuel injectors that affect cylinder fueling.
Further improvements in fuel economy can be achieved in engines configured to vary the effective displacement of the engine by stopping the delivery of fuel to certain selected cylinders in an indexed cylinder firing pattern. One example of selective cylinder non-fueling is shown by Tripathi et al. in U.S. Pat. No. 8,651,091. Therein, an engine fuel controller may continuously rotate which particular cylinders are fueled, which cylinders are not fueled, and how many cylinder events and/or engine cycles for which the pattern is continued. In addition, optionally individual valve mechanisms of each cylinder not fueled may be selectively deactivated, or maintained operating so that the cylinders pump unfueled cylinder charge to the exhaust. By avoiding fuel delivery to selected cylinders, the active cylinders with fuel delivery can be operated nearer a desired efficiency, increasing the overall operating efficiency of the engine. By varying the identity and number of cylinders selectively not fueled, a large range of engine displacement options may be possible.
However, the inventors herein have identified a potential issue with such engine systems. In particular, an estimation of cylinder air charge may be difficult to compute due to the numerous firing patterns that an engine controller may run. In systems, such as two-position VDE engine systems with, for example, four-cylinder and eight-cylinder modes, two sets of look-up tables may be generated based on measured data from the two modes. Each set of look-up tables may be used when the engine is operating in the respective mode.
For selective non-fueling operation, however, the wide variety of modes available would necessitate an equal number of look-up tables, which may results in over-complexity such that it is not ever possible to have overall accurate estimation of cylinder air charge. An accurate estimation of cylinder air charge is useful in determining appropriate fuel injection amounts, spark timing, torque estimates, and so on. Furthermore, cylinder air charge estimates may be used to estimate and schedule many other variables, such as air-fuel ratio. As a result, inaccurate cylinder air charge estimates may result in a degraded efficiency of engine operation and may adversely affect emission control.
In one example, some of the issues above may be addressed by a method for an engine that comprises adjusting an air charge estimate for a first cylinder based on a firing pattern of another, second, cylinder. In this way, cylinder air charge estimates may be improved in real-time for an arbitrary cylinder firing pattern, thereby improving engine efficiency during selective cylinder non-fueling/firing operation.
In another example, a method for an engine comprises adjusting an air charge estimate for a first cylinder by a first amount responsive to a second cylinder not firing during a current cycle or a previous cycle, adjusting the air charge estimate by a second amount responsive to the second cylinder firing during the current cycle and not firing during the previous cycle, adjusting the air charge estimate by a third amount responsive to the second cylinder not firing in the current cycle and firing during the previous cycle, and not adjusting the air charge estimate otherwise. In this way, cylinder air charge estimates may be improved, which in turn may improve fuel economy and emission control.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.